Transcript MICROEVOLUTION

MICROEVOLUTION
The Genetic Basis for Evolution
The Theory of Evolution by
Natural Selection
• Populations of
organisms have the
capacity to change over
time, generally in
response to changes in
their supporting
environment.
Microevolution
We know….
Alleles
Genotype
Phenotype
Form
Adaptation to Environment,
Evolution
Microevolution
• So in order to REALLY figure out
Evolutionary Change, we’ve got to back
up…
Alleles
Genotype
Phenotype
Form
• Microevolution = Changes in allele (gene)
frequencies over time (in populations).
Microevolution
• GENE POOL:
– All of the alleles at all gene
loci in all individuals of a
population.
– The Alleles/Genes, not the
Individuals.
– The Genotypes, not the
Phenotypes.
Microevolution
• Please read pp. 454-458…
• The Hardy-Weinberg Theorem:
p2 + 2pq +q2 = 1
• Where p = allele frequency 1
• Where q = allele frequency 2
Microevolution
Aa
• REVIEW
– Alleles
– Diploid/Haploid
– Homozygous
– Heterozygous
– Dominant/Recessive
AA
aa
Aa
aa
Aa
AA
aa
AA
• Frequency of alleles =
total # of alleles of one type
total # of alleles of all types
Microevolution
• Frequency of alleles=
A= 9/22 = 0.41 = 41%
a = 13/22 = 0.59 = 59%
Aa
aa
AA aa
Aa
aa
Aa
AA
aa
aa
AA
• Traditionally, in Hardy-Weinberg:
p represents the frequency of one allele &
q represents the frequency of the other allele.
Hardy-Weinberg Theorem
• Says that frequencies of alleles in a
population remain constant over time.
• This depends on the inheritance of
variation introduced during segregation &
recombination (Mendelian Genetics).
• Defines when a population is NOT
evolving.
Hardy-Weinberg Theorem
• Organisms donate gametes to the next
generation randomly.
• Organisms mate with each other
randomly.
• SO, we can predict the genotypes of
offspring…
Hardy-Weinberg Theorem
• Frequency of alleles=
A= 9/22 = 0.41 = 41%
a = 13/22 = 0.59 = 59%
Aa
aa
AA aa
Aa
aa
Aa
AA
aa
aa
AA
• The chance of a gamete carrying each allele is the same.
– So, the chance of having offspring =
For AA, p x p = p2 = .41x.41 = .168 = 16.8%
For aa, q x q = q2 = .59 x .59 = .348 = 34.8%
For Aa, p x q = .24 = 24.2% AND q x p = .24 = 24.2%
Hardy-Weinberg Equilibrium
• This set of relationships (predictions) can
be summarized as:
p2 + 2pq +q2 = 1
• In any population, Hardy-Weinberg
predicts the frequencies of alleles in each
generation, which always = 1.
• Sort of like a deck of cards…
Hardy-Weinberg Theorem
•
Assumptions for Hardy-Weinberg to hold
(populations NOT evolving):
1.
2.
3.
4.
5.
Large Populations
No Migration
No Mutation
Random Mating
No Natural Selection
– BUT, in Natural Populations, these 5
assumptions are violated…
Agents of Microevolutionary
Change
•
Processes that elicit changes in allele
frequencies in populations:
1.
2.
3.
4.
5.
Mutation
Gene Flow
Non-Random Mating
Genetic Drift
Natural Selection
Agents of Microevolutionary
Change
1. Mutation: A random heritable change in
the DNA.
– Only source of NEW alleles in a population.
– Actually rather common!
– Can be:
•
•
•
Harmful (most – we’re complex), these do not
remain in the gene pool and so are rare (selected
against!).
Neutral (many), masked, occur in genes not
essential for survival, often passed on.
Beneficial (rare – we’re complex), these are the
key to better-adapting to changing environments.
Agents of Microevolutionary
Change
2. Gene Flow: New individuals enter or exit
a population (Migration).
•
•
Introduces the possibility of new allele
combinations.
Often has a homogenizing (mixing) effect on
the population – offering stability.
Agents of Microevolutionary
Change
3. Non-Random Mating: Mating &
offspring production by individuals with
specific phenotypes (genotypes).
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•
•
A.K.A. “Assortive Mating” or “Preferential
Mating” (Not random).
Results in accumulation of certain traits in
the population (sometimes reduction of
heterozygotes).
Examples: Mate selection in white-tailed
deer (“Sexual Selection”), tall Men & tall
women, etc.
Agents of Microevolutionary
Change
4. Genetic Drift: Changes in allele frequency
due to chance.
•
•
Often due to natural disasters or major
catastrophes.
Especially in small populations.
•
•
Bottleneck Effect: loss of certain alleles as a
population dwindles, reduction in allelic diversity
with a smaller and smaller population.
Founder Effect: small population with a specific
and limited gene pool is separated from the main
population.
Agents of Microevolutionary
Change
5. Natural Selection: Differential survival &
reproduction of individuals in a
population.
1. Stabilizing (the extremes of the population are selected
against in favor of the average/most common)
2. Disruptive (the average/most common individuals of the
population are selected against in favor of the extremes)
3. Directional (one extreme or the other of the population are
selected against)
Agents of Microevolutionary
Change
5. Natural Selection
(Peppered Moths)
Natural Selection
• Hardy Weinberg & Natural Selection:
http://ats.doit.wisc.edu/biology/ev/ns/t3.htm
The “Modern Synthesis”
• Incorporation of population
genetics into natural
selection.
• Biochemistry
– Genes
• DNA
• RNA
– Amino Acids
T. Dobzhansky
Next?
• What is a species?